In order to increase the driving efficiency and safety and to reduce wear of the wheel brakes, primarily heavy goods vehicles are, according to prior art, equipped with retarder devices. A retarder device is a hydrodynamic auxiliary brake that brakes the car's driveline with the help of resistance exercised by a blade system, which impels an amount of liquid in a workspace. Braking energy absorbed by the retarder device transitions in operation into heat energy in the liquid, which in turn is cooled down by a cooling system in the vehicle.
The braking torque is controlled through control of a pressure prevailing in the workspace between the blades belonging partly to the retarder device's stator, and partly to its rotor. When the retarder device is not used, the workspace is essentially depressurized. However, a certain drag loss is created in the retarder device since it still rotates with the driveline. From a fuel-saving perspective it is important to keep this drag loss as small as possible.
A method to avoid the drag loss is to mechanically disconnect the retarder device from the driveline when it is not used. This is possible with the help of a coupling, such as a synchromesh or a disc clutch. When the retarder device is then to be connected to be used for braking, the retarder device's rotor must be accelerated quickly and synchronized with the driveline's engine speed. This must happen within fractions of a second, and the acceleration torque produced must be minimized in order to avoid uncomfortable jerks to be generated in the vehicle.
In a retarder device, which uses oil as a working medium, this is fully possible thanks to a low acceleration resistance, which is achieved since the blade system is filled with air instead of oil during this acceleration phase.
In order to minimize minimise the number of components and systems in the vehicle, it is advantageous if the vehicle's ordinary coolant, which typically contains a mixtures of water and glycol, may be used as a working medium. A retarder device applying this principle is called a water decelerator.
In a water retarder, however, it is problematic to aerate the blade system during acceleration, since air that comes into contact with the coolant may partly follow the latter and have an adverse impact on other components in the vehicle with which the coolant comes into contact. For this reason, partly water-filled water retarders are operated also when they are disconnected, entailing a relatively high acceleration torque.
For this reason, more complex connecting components are required and uncomfortable jerks may nevertheless to some extent still arise in the vehicle at connection.
As examples of the technology's background, U.S. Pat. No. 2,287,130, US20060909971, WO2010034493, US2007102251 and DE10054078 may be mentioned.